FEATURE

Sky-high laser processing

Stephen Mounsey examines novel aerospace materials, and the innovative processing techniques that their introduction has necessitated

A modern aircraft is a magnificent piece of work, and perhaps the most technically impressive parts are its engines. In commercial aviation, a continuing drive for fuel economy, along with pressure from conscientious environmentalists, has resulted in a move towards more efficient engines. Noise pollution generated by the expanding airports has also gained attention, and new engines are designed to be as quiet as possible. Understandably, safety is the primary design constraint that engineers work towards, and the highest standards must be met at every stage. The advancements in materials science, which have been necessary to meet such stringent criteria, have forced manufacturers to innovate to just as high a standard. High-powered lasers have offered the only viable option for achieving the complexity and precision the industry requires, and in many of these cases novel techniques have been developed.

Why such advanced materials?

In older jet engines, called turbojets, all of the engine’s thrust is generated by the accelerated exhaust gases. Turbojets are fuel inefficient, and are seldom used in modern aircraft. The modern turbofan engine uses larger fan blades, which divert cold air around the combustion chamber, allowing slower air to mix with the high-speed air from the exhaust. This spreads the engine’s thrust across a wider area, resulting in greater efficiency. High-speed jets use a low-bypass configuration, in which most of the thrust is generated by the jet effect. More efficient and quieter commercial aircraft use a high-bypass engine, in which most of the thrust is generated by the main fan blades.

The turbine blades, which operate in the hot part of the engine, immediately behind the combustion chamber, must capture the energy of the high-velocity exhaust gases, in order to generate efficient thrust. The thermodynamics of the system are such that the engine achieves higher fuel efficiency at higher temperatures, so blades are made to operate at the highest temperatures they can stand. A temperature increase of 10 or 20°C can equate to efficiency changes of 5 or 10 per cent, even at temperatures up to 1,100°C.

At these temperatures, problems present themselves: Most alloys rapidly become a pool of liquid, and the few alloys that are able to remain solid will fall victim to creep – the high-temperature, diffusion-driven process by which alloys deform under the centrifugal forces of the spinning shaft. Furthermore, the alloys must have good corrosion resistance and mechanical strength in order to survive the heat and forces involved. Modern turbine blades make use of so-called superalloys, which are nickel-based with additions of cobalt, titanium and aluminium, along with traces of various other metals to help increase resistance to corrosion and creep. Creep occurs at the boundaries between the microscopic grains in a metal, and so the blades are cast as single crystal with no grain boundaries. By the time it is completed, a single turbine blade can cost many thousands of pounds.

Despite the impressive properties of the superalloys, turbine blades are still operated at temperatures well above their melting point. A film of cool air is passed over each blade in order to insulate the component from the hot gases passing over it. This film is achieved by drilling holes in the component, through which cold air is fed. There may be hundreds of holes drilled per blade, each one less than a millimetre in diameter; for example, the F-135 engine in the Lightning-II fighter aircraft has over two million such holes drilled in the hot (rear) part of it.

A typical air hole might be 0.5mm in diameter, and the manufacturers may specify a ±10 per cent air-flow tolerance, which scales with the area of the hole. The diameter of the hole must therefore be accurate to five per cent, i.e. to 25μm. Laser drilling generally uses a pulsed Nd:YAG laser, with average power of 70-200W and relatively long pulses of 0.5-1.5ms.

Peter Thompson, technical director at Laserdyne Systems (a division of Prima North America specialising in laser drilling for aircraft components), explains the factors necessitating a pulsed laser approach. ‘The manufacturer wants to remove material from the workpiece,’ he says, ‘but then again, they’re constrained by the need to achieve the metallurgy that the customer wants, and so we have two conflicting requirements; we have to have high enough power to remove the material, but at the same time we need low enough power not to put more heat into the component than we really have to. The way of meeting both these requirements is to use a pulsed laser.’

Turbine blades can operate at very high temperatures.

The difficulty in reaching the strict standards for the metallurgy of the components, including control of the thickness of any re-cast, how much cracking is acceptable, and the properties of the heat-affected zone, are made all the more challenging by the unusual material used. ‘Optimising the process to drill holes in single crystal components has been quite challenging, especially when we have to think about producing the correct metallurgy,’ says Thompson. ‘We have to modulate the pulse energy, pulse width, peak power, and pulse frequency. We also have variable parameters in terms of the assist gas used. Typically, we optimise these processes through a series of experiments.’

Controlling spot size

In most established laser drilling systems, hole size is controlled by drilling in a de-focused manner. The parameters that give the required metallurgy are determined by experimentation, and the laser beam is focused at a point below the workpiece. Movement in the focal axis is used to obtain a laser spot of desired diameter for the required hole.

Mark Barry, vice president for sales and marketing at Prima North America, explains that this approach requires an experienced operator, as it takes good judgment to achieve the correct spot size. ‘We’ve been doing it this way for 30 years now,’ he says. ‘The biggest problem we face when trying to integrate laser technology throughout the factory is that this technique currently requires a very skilled operator. The operator has to understand where the work piece is within the range of this defocused beam. Every time an adjustment of the hole size is required, it’s purely a judgment on the part of the operator; we’ve tried for years to write algorithms that will predict what axial movements will do to hole size, and it just doesn’t work.’ Barry believes that insufficient research effort is being put into taking out this operator dependency: ‘Everybody is excited about different aspects of the technology; different rods, different powers, different methods of transmission. The problem, however, is the lack of process control,’ he says.

The cost of a laser operator could be significant, as laser processing is often one of the last processes in the manufacture of the component. ‘We get to the stage where there’s a £150k component underneath the laser, and the technique relies upon an operator’s judgments, based on nothing more than experience. It’s a horrible manufacturing process, in need of more automation,’ says Barry.

One way of achieving that automation may exist in the form of at-focus drilling. Here, instead of defocusing the beam, an optical arrangement known as a variable spot module (or VSM) is used to change the diameter of the raw beam, prior to the focusing optics. The beam diameter at focus is therefore tunable, as well as the depth of focus. In addition, the beam’s quality is maintained, and so the parameters necessary to create the correct metallurgy need not be adjusted. According to Thompson, at-focus drilling is capable of producing holes that closely match the theoretical beam diameter, allowing a high degree of precision, which is less dependent on operator expertise.

Some early designs of air-cooled turbine blades had up to 2,000 holes in them. Manufacturers quickly reached the limits of this kind of diffusion cooling, and it was then that coatings began to become necessary. Through the use of thermal barrier coatings (TBCs), which can be either ceramic or metallic, the manufacturers have achieved what Barry describes as a ‘quantum leap in the temperatures the engines can be run at’. When drilling through TBCs, delamination of the coatings becomes a concern, and metallurgical standards are still as important as in the uncoated component. Coatings tend to have a very high melting point, and the differential between that of the metal and coating can lead to problems. In practice, Barry states that a greater awareness of the laser parameters is required, and that the obtainable tolerances may be reduced as a result.

In future, turbine blades will have fewer holes, but they will be of more complex design. By controlling air-flow more precisely, the holes will be able to create a cooling effect comparable to current designs while requiring less cooling air (which is provided at an efficiency-cost to the engine). This kind of precision simply could not be achieved without laser drilling.

As well as looking at ways of making the engines more efficient, manufacturers of larger commercial jets (i.e. Boeing in the US and Airbus in France) are beginning to use lightweight composite materials in their airframes. Chiefly, these materials are laminated fibre-reinforced composites (usually carbon fibres reinforcing a polymer matrix – CFRP for short), which offer material properties superior in many ways to metallic alternatives. Metal matrix composites such as Glare (a glass fibre reinforced aluminium alloy) have also been used. Currently, CFRPs are processed by the aerospace industry using traditional machine tools, or by way of water-jets. Laser processing is yet to be used for processing of composites in the industry.

Dr Martin Sharp and Dr Paul French of the General Engineering Research Institute (GERI) at Liverpool John Moores University, recognised that if lasers were going to play any part in the future of airframe construction, techniques of laser material processing would have to be developed for aerospace composites, especially CFRPs. The team approached a number of UK-based aerospace companies, asking them for samples of aerospace-grade composite materials, before going about seeing how the various materials react to laser processing. Two processes were examined; laser cutting of laminated composites, and laser surface microtexturing in order to give the composite more wettability, improving the spreading of adhesives and increasing the strength of any joints.

With respect to laser cutting, the team used JK200 fibre laser, produced by GSI in Rugby, UK. The laser has a maximum power of 200W CW, focused at the workpiece to spot diameter of 20-25μm. ‘Initially we started with thin samples to feel our way with this new material,’ says French, ‘and the results looked promising. We are working to solve the surface damage problem and we are working towards laser machining thicker sheets. [Very recently] we successfully laser-cut 7mm holes into a 3mm-thick piece of CFRP. Based on these results we believe that processing CFRP material 6-10mm thick shouldn’t be a problem, while still maintaining a good edge quality.’ French cites the target application as being aerospace companies looking to machine holes into 30-50mm thick CFRP, and with the research the group has undertaken so far, coupled with the increasing availability of high-powered fibre lasers, he thinks these goals are readily attainable. The technique causes superficial damage to the top layers, but the cut is clean and free of the delamination that accompanies water-jet or mechanical cutting.

The group’s laser microtexturing work was carried out using a FemtoPower 1060-4μJ-pp ultrafast fibre laser supplied by Fianium, based in Southampton, UK. The surface of a composite was textured prior to applying adhesive for bonding. French states that the results of the tests have been very encouraging; the surface wetness measured has been far superior to that produced by mechanical abrading, while the associated damage due to the process is significantly less. In addition, the joint produced by the adhesive is stronger than in the surface abrading case.

The team has already extended this work to processing glass fibre composites (GFCs), and French says the aerospace company the team has been working with find the results promising. ‘We have been told by the industry that the cut in GFC looks superior to water-jet, but that it leaves a black deposit on the cut surface. The industry doesn’t like this, and so we are working on reducing this.’

Regardless of what direction the aerospace industry takes in the coming years, experience has shown that many of its developments depend on innovation in advanced materials. These new materials will often necessitate development of novel processing techniques, and laser-based processes have thus-far proven the most versatile solutions across a diverse range of applications.